1. Field of the Invention
This invention relates to the manufacture of semiconductor devices and particularly to a method for locating and repairing latent pinholes, cracks and microscopic areas of low breakdown voltage in dielectric layers prior to completion of the devices.
2. Prior Art
The detection and elimination of pinholes in dielectric materials used in the manufacture of semiconductor devices is crucial to maximizing device yield and minimizing device cost. A number of techniques have been suggested for at least partially eliminating pinholes. One technique, which recognizes that defects in masks which might cause pinholes have a higher probability of falling over the field of the device than over the active areas of the device, forms thick oxide over the field of the device. As a result, pinholes are unlikely to form through the thick field oxide. A second technique forms at least two different dielectric layers in sequence, one layer directly adjacent the other, so that a pinhole in one dielectric layer is highly probable of being covered by a non-pinholed region of the other dielectric layer.
In a silicon oxide-silicon nitride two-layer dielectric the self-healing of existing pinholes in the silicon nitride layer by a subsequent oxidation is well known. This prior art technique relies on the fact that the oxidation rate of silicon nitride is extremely slow compared to that of silicon and works only when the pinhole in the silicon nitride extends through the nitride to the silicon oxide. However, when latent defects are present in an oxide-nitride layer such that the nitride film is continuous throughout the defective region but thinner than desired, or the oxide underlying the nitride has a pinhole or is thinner than desired, or is defective due to contamination, or the nitride overlying the oxide has a defect due to contamination, or any combination of these defects, the prior art method does not work since the underlying substrate is not exposed and the oxidation rate of the nitride itself is extremely small. Such latent defects as described above often cause local low breakdown voltages and sometimes cause a total malfunction of part of an integrated circuit, or a leakage sufficient to prevent meeting required test and reliability specifications. As devices become smaller, such low breakdown voltages will have a greater likelihood of adversely impacting device performance. For example, channel contamination, particulate contamination, thin regions and protrusions due to precipitates in the substrate all locally enhance the electric field and thus can cause abnormal performance where the adjacent dielectric is thinner than designed.
The prior art has no method of detecting and correcting for the above described latent defects prior to completion of the circuit. This invention provides a remedy for these defects which is both easily implemented and relatively effective.